Continuously accessible optical cable

ABSTRACT

The invention concerns an optical cable ( 1 ) with continuous accessibility, comprising a closed protective sheath ( 6 ) preferably oval-shaped enclosing a cavity ( 5 ) preferably oval-shaped having in cross-section two substantially perpendicular axes intersecting at the centre of the cavity, and at least two optical fibres ( 3 ) optionally assembled in at least two modules ( 2 ), and arranged such that they take up the major part of the cavity in an axis but allow significant clearance in the other axis of the cavity. The fibres are preferably arranged in the longitudinal axis. The inventive optical cable is continuously accessible as a result of its preferably oval general shape, its two preferred axes one for an easy curvature and accessibility to modules distributed along the large dimension of its central cell and the other for a significant clearance of the modules which also facilitates extraction and provides the cable with good thermal and mechanical properties.

[0001] The present invention relates to an optical cable (also called anoptical fiber cable) with continuous accessibility, particularlysuitable for local subscriber loops and for internal cabling, providingvery simple accessibility at any location and thus providing a very lowcost branch-off without having to use a branch connection box.

[0002] Many types of optical cables are known, making it possible tosupply local networks or the cabling of buildings. In the majority ofcases, these cables have a very dense structure. They comprise arelatively large number of fibers, and they are branched off using abranch connection box. It is by virtue of this branch connection boxthat after an often lengthy and tricky operation of removing the sheathfrom the cable over relatively great lengths, then of distributing thefibers, that it is possible to branch off one or more fibers. However,this operation is expensive and tricky. In addition, it is not withoutrisk for the unbranched-off fibers which, in the majority of cases, aremoreover high datarate carriers, that is to say ones having a largenumber of links transmitting voice, data or images. In addition, becauseof their architecture, these techniques are very often synonymous withdistributed systems (for the cabling of buildings in particular),requiring the placement of numerous elementary cables, not to mentionthe presence of optoelectronic end devices. These distributed systemsthus have the major drawback of leading to extremely high laying costs,which, for optical systems, generates a negative end result comparedwith the known copper systems.

[0003] To compensate for this major drawback, and to reduce the cost ofinstallation, the majority of optical cable solutions developed to datetend toward cables which are as dense as possible, based on eitherstacks of ribbons, or on cylindrical modules of very small dimensions.Each ribbon or each cylindrical module may, for example, contain from 4to 12 fibers, or even more. All of these ribbons or of these cylindricalmodules are packed as closely as possible in a protective sheathpossibly provided with reinforcements, according to the functionalspecifications of the cable. The sought-after aim is to obtain maximumdensity while retaining the appropriate mechanical and thermalproperties for the optical cable.

[0004] The density thus obtained in the highest performance cables ofthe prior art allows them to be laid in microconduits, for example byblowing or pushing or pulling. However, these cables have the drawbackof being quite difficult to access thereby making the tap-off operation,that is to say the branching-off of one module among n or of one fiberamong n, expensive and tricky. They are therefore better suited todistributed techniques where the breakout made necessary takes place ina box designed to this end. In addition, the dense and cylindricalstructure of the jacket of such cables requires an internal “cabling” ofthe optical elements (modules or optical fibers). This internal cablingis an assembly of optical elements with a given pitch either in acontinuous helix (helical cabling), or in SZ (SZ cabling), so as toguarantee the integrity of the fibers when winding the cables.

[0005] Thus, in these known techniques, in order to operate correctlyfrom the mechanical and thermal point of view and in order to complywith the radii of curvature, the relatively dense cable structuresrequire “cabling” of this sort, which only increases the difficulty ofaccess to the fibers and in particular makes it necessary for all thesheath to be removed over considerable lengths so as to allow theoperator to handle the bundles with enough flexibility to access thesought-after module and, even more difficult, the sought-after fiber. Inaddition, cabling of this sort decreases the speed of cablemaking, andincreases the costs. It can therefore be seen that, whatever theirshape, oval, cylindrical, or other, none of these structures has beendesigned for continuous and easy accessibility of the fibers.

[0006] There is therefore a need for a cable designed to allowcontinuous and easy accessibility, therefore allowing easy branching,while remaining a very economical solution and installation system.Solving this problem is all the more significant since placing severalcables in parallel, for example in order to supply several workstationsinside a building, remains an expensive operation which places opticalsystems at a disadvantage with regard to copper.

[0007] A branchable optical cable is known, the principle of which is toplace cable elements in a sheath or profile in the shape of a U open onone side, which may then be opened out right along the cable, at anylocation on the cable, and thus allow access to the cable elements. Thiscable was the subject matter of patent application FR 99 13271 filed bythe applicant. However, a cable of this sort involves the specialmanufacture of cable elements and the use of a profile which may proveheavy and bulky and quite difficult to utilize, especially in terms ofits storage on a cable drum, its laying and its accessibility.

[0008] The applicant has therefore developed an optical cable withcontinuous and easy accessibility, therefore making it possible toextract very easily from it, at any location, an optical elementprotecting one or more optical fibers, intended to supply a room or aregion comprising several workstations. The cable developed by theapplicant offers tensile strength and crush resistance properties whichenable easy installation, for example in a conduit or in cable trays, inthe latter case with an increased ability to follow localized curves andwith a density which remains reasonable. Because of its structure, thecable which is subject of the invention also has excellent thermalbehavior and can be wound with no problem on cable drums, withoutrequiring cabling of the elementary optical elements inside the sheath.

[0009] The subject of the invention is therefore an optical cable withcontinuous accessibility, comprising a closed protective sheathsurrounding a cavity having in cross section two substantiallyperpendicular axes intersecting at the center of the cavity, and atleast two optical fibers arranged such that they occupy the greater partof the cavity along one axis but that they allow a sizeable clearancealong the other axis of the cavity. The optical fibers can be organizedinto at least two optical elements (also called optical modules). Theoptical fibers, or optical modules, therefore form a sort of sheet,which is relatively loose or not depending on the number of fibers ormodules. This arrangement of the cable according to the invention doesnot exclude some superposition of the fibers or of the modules withinthe sheet. Preferably, one of the axes is longer than the other axis.The optical elements are preferably positioned along this long axis, andtherefore the clearance is left in the short axis. The clearance left inone of the axes is considerable with respect to the usual clearance of adense optical cable structure, which is generally reduced to the strictminimum. This clearance therefore allows variations, called overlength,of the order of one percent or of a few percent (overlengths consideredin per thousand in dense structures). Moreover, the sheet of fibers orof modules may move by translation in the cavity while remaining on thesame axis (on the long axis in the case of an ovalized cavity).

[0010] The cavity has any shape, for example circular or oval orovalized (that is to say between an oval and a rectangle), preferablyovalized. The sheath has any external shape, for example circular oroval or ovalized, preferably ovalized. In a particularly preferredembodiment, sheath and cavity are of ovalized shape, and their long axesare coincident.

[0011] In some embodiments, the sheath may comprise one or moremechanical reinforcing members, preferably positioned on either side ofthe cavity, again preferably along the long axis. These reinforcingmembers will preferably be of the nonmetallic glass-epoxy type, but mayalso be metallic. In an application of cable run type and overrelatively short lengths, these reinforcing members may prove not to beindispensable making the production of the cable even more economical.

[0012] Thus, contrary to the teaching of the prior art, which encouragesthe perpetual search for an ever increasing density, the density of thecable which is the subject of the invention is deliberately less thanthe density that could be reached if the cavity was full. However, acable of this type retains enough density to be used simply ininstallations of the conduit or cable-tray type, while offering someclearance in the cavity, facilitating the accessibility of the opticalelements therefore of the optical fibers, and this by means of anoptimized filling, a compromise between maximum filling and too low adensity.

[0013] The preferably ovalized external shape of the cable according tothe invention, together with the possible reinforcing members, confer onsaid cable a curving behavior which favors the long axis of the sheathpreferably coincident with the long axis of the cavity, and thereforeensures winding along this long axis taking advantage of theconsiderable clearance in the short axis. This makes it possible toavoid “cabling” of the optical elements, which may therefore be placedlengthwise without twisting. This also offers some flexibility to thecable, in spite of its dimensions, which makes it possible to apply thecable easily to the curves, which are sometimes severe, of the cabletrays. Moreover, the cable obtained has very good thermal and mechanicalbehavior while offering very simple accessibility by localized openingparallel to the long axis and visibility of the possibly colored opticalcable elements laid lengthwise in the cavity. The curvature favoredalong the long axis of the external ovalized part and the reasonabledensity of the cable also make easy storage of significant lengthspossible on a drum before laying and facilitate installation, andespecially the curvatures required by laying in cable trays.

[0014] Thus, the cable which is the subject of the inventionadvantageously contains very dense optical modules, for example madefrom fibers surrounded with a thin film in order to form elementarymodules which may range, for example, from two to twelve fibersdepending on the intended application. Within one and the same cable, itis possible to mix modules (for example 12) of different capacities.Moreover, such elements are perfectly known and mastered in order tobring together fibers (conventionally of 250 μm diameter) in a minimumspace with very simple accessibility given the ease of baring orstripping the thin film. These modules are modules used in cablemakingand therefore belong to known techniques for producing highly compactcables. These elements are thus placed lengthwise in the cavity of thecable which is the subject of the invention. The considerable clearanceencountered by these elements along an axis, preferably the short axis,of the cavity makes it possible to achieve very high cabling rates andtherefore to produce a cable quickly and very economically. They have alow coefficient of friction which makes it possible to extract them froma row of modules without any difficulty, that is to say without a forcedetrimental to the behavior or to the strength of the fibers, includingover lengths which may reach several meters.

[0015] The protective sheath of the cable according to the invention andits cavity have dimensions suitable for the use envisioned. By way ofexample, the dimensions of the sheath may be from 10 to 15 mm for thelong axis and from 6 to 8 mm for the short axis (for an ovalizedsheath). By way of example also, the dimensions of the cavity may befrom to 5 to 9 mm for the long axis and from 2 to 4 mm for the shortaxis. The optical elements or modules positioned in the cavity may havea diameter or a longer dimension in section of about 0.8 mm to 1.3 mm.

[0016] The optical elements therefore have a certain degree of freedomwithin the cavity, making it possible to remove stresses on theseelementary modules, whether this is after an extension of the sheathunder a certain tension or under the effect of an expansion, or after acontraction due to low temperatures.

[0017] For this, the cablemaking process, well known moreover in thefield of cablemaking, provides, when making the protective sheath andinserting fibers, an initial overlength which is small but sufficientfor there to be no actual stresses when the cable is under tension orexpanding.

[0018] This high degree of freedom along one axis of the cavity allowsthe optical modules (or fibers) to spread out flat along the other axisof the cavity, and offers, as specified above, two considerableadvantages. On the one hand, the possibility of lengthwise cabling ofthe modules, leading to a very simple process, allows high and veryeconomical production rates. On the other hand, the “sheet” of modules(or fibers) is easily accessible both because of the visibility of themodules and because of the considerable clearance of these same modulesin the cavity making them extremely easy to extract including over greatlengths. The branching-off of a module (or of a fiber) is carried outwithout stresses that could damage the module itself (or the fiber), andwithout an interaction that could damage the nearby modules (or fibers),which is essential for high datarate links and for interventions whichmay take place while some fibers or some modules are already in service.

[0019] The material used for making the sheath is preferablyinexpensive, and makes it possible to obtain small radii of curvature,smaller radii of curvature being made possible the greater the lateralclearance of the modules in the cavity, and effective protection of thecable especially against shocks and crushing.

[0020] A suitable material may be chosen among elastomers of the naturalrubber type, styrene/butylene/styrene andstyrene/ethylene-butylene/styrene copolymers, flexible formulations fromthe elastomeric range, composed of polymers such as ethylene/methylacrylate, ethylene/ethyl acrylate and ethylene/butyl acrylatecopolymers, and ethylene/vinyl acetate copolymers, these polymerspossibly being combined with flame-retardant fillers and antioxidantadditives; copolymers of the polyesterether type such as the ARNITEL®copolymer manufactured by DSM or the HYTREL® copolymer manufactured byDuPont de Nemours. Preferably, it will be one or more halogen-freeflame-retarded polyolefins, of the type commonly used in sheathingcables placed in a building. For external use, it is possible toenvision producing a cable with a cavity accommodating the modules,surrounded with a sheath made, for example, of PVC, coated with a mediumor high density polyethylene-type jacket more suited to pullingtechniques in a conduit and comprising reinforcing members.

[0021] In one embodiment, the sheath may have one or more coloredthreads, obtained for example by coextrusion. The role of these threadsis to indicate the regions where the windows can be drilled and, also,to identify the optical cable while giving it an attractive appearance.The windows may be opened on either side of the sheet of fibers ormodules. The presence of these colored threads thereby facilitates theopening by the installer.

[0022] In another embodiment, the sheath may have one or more fractureinitiators marked to a greater or lesser a degree and possibly colored,for example in the hollow of the initiator, possibly in combination withone or more colored threads. Such initiators further facilitate theopening of the windows. However, they tend to embrittle the structure ofthe cable. An embodiment of this sort is therefore better suited tocables subject to few stresses, for example in the case of buildingswith cables laid on a cable tray.

[0023] In another embodiment, the sheath has a region of lowerthickness, between two guides, so as to facilitate the opening ofwindows at this lower thickness region.

[0024] In the case of cables subject to considerable stresses, forexample those placed by pulling, blowing or pushing in a conduit and onan external site, it will be preferred to use a cable comprising, inaddition to the usual sheath, an additional external jacket.

[0025] In this case, the external jacket may comprise one or morecolored threads, and/or one or more fracture initiators, and/or oneregion of lower thickness. This jacket may also contain reinforcingmembers. The material of this jacket may be of the medium- orhigh-density polyethylene type. The sheath and the jacket may bemanufactured by coextrusion or, more easily, by successive extrusion ofthe sheath, for example made of PVC, then of the jacket.

[0026] When installing a cable according to the invention, it istherefore seen that it is possible to access, in a continuous manner,that is to say at any point on the cable and at any time, in anextremely simple manner, the optical modules or elements present in thecable.

[0027] For example, it is possible to open part of the cable along thelong axis over a very short length, and to remove part of the sheathusing a tool specially adapted to the shape and to the dimensions of thecable, thereby producing in this way a first window for access to thelocation intended for the branching-off. Because of its shape, the cableacts as a guide for the tool, which may comprise for example one or twocutting blades. The opening is without risk for the modules or thefibers because of their clearance in the cavity. It is possible torepeat the operation downstream, at a distance from the first windowchosen according to the desired branching length, a length which mayrange, for example, from a few tens of centimeters to several meters,and thus to form a second access window. This is then enough to accessthe module chosen in the second window, to cut it, then to extract itthrough the first window, that is to say the window chosen for thebranching.

[0028] An extraction of this sort is made possible and easy by virtue ofthe clearance of the modules or of the fibers in the cavity, by virtueof the low coefficient of friction of the material forming their sheathor film, and finally by virtue of the fact that the modules or fibersare laid lengthwise therefore without cabling. Moreover, the windows maybe opened only over short lengths, typically a few centimeters.

[0029] After the branching, it is possible to close the windows again byattaching (for example by adhesive bonding) a lid which acts as a patchand makes it possible to establish better protection of the modules orfibers inside the cable, a cable which has retained its mechanicalproperties throughout the branching operation. Moreover, it is possibleto place the branched-off fiber or module inside a corrugated sheath ofsuitable dimensions or any type of prerouted sheath depending on theenvisioned use, without having to use intermediate splicings.

[0030] Of course, in certain applications, it is possible to envisioncables of smaller dimensions with less fiber capacity, by placingpossibly colored optical fibers directly in the cavity rather thanmodules. If modules are used, it is seen that there are manypossibilities depending on the aims of the cabling project, eachelementary module possibly comprising 2, 4, 6, 8 or 12 fibers forexample, offering cable capacities ranging for example from 24 to 96fibers or even more, depending on the number of modules.

[0031] According to the chosen technique, it will be preferred to workwith low-capacity modules in order to serve only a single office forexample, or it will be preferred to work with higher capacity modules soas to serve a dense region consisting of four, eight or twelve officesor one office containing several workstations.

[0032] It is therefore seen that the cable which is the subject of theinvention may meet the needs of many engineering choices while retainingits intrinsic qualities of continuous accessibility and of considerablesaving on the installation costs.

[0033] Of course, this principle of a cable with continuousaccessibility is based on the fact that the cost of optical fibers hassubstantially decreased in the light of decisive productivity steps inmodern production processes, and that the technique of continuousbranching assumes, except when assuming looping of the links, that theparts of the fibers downstream of the branch are lost for the use inquestion. On the other hand, it is possible to use these fiberssubsequently either to loop a link or to create intermediate links. Thefact of losing lengths of fibers proves in any case to be markedly moreeconomical, particularly when using monomode fibers, than laying seriesof short cables and producing a structure of the distributed type.

[0034] The fast and substantial increase of datarates formultimedia-type applications moreover strongly favors the direct use ofmonomode fibers, including in the vertical and horizontal parts ofbusiness cabling, provided that very bottom of the range and very lowcost optoelectronic modules are attached for such use. The cable whichis the subject of the invention therefore opens a new way of cabling abuilding or of cabling small town networks, loops etc., which is veryeconomical and very flexible to use, making it possible to envisioncompetitive optical solutions with respect to the successive solutionsusing copper media, while offering the user a medium capable ofaccepting considerable datarates.

[0035] Other characteristics and advantages of the invention will appearfurther on reading the description which follows of one particularnonlimiting embodiment, in relation with the figures in which:

[0036]FIG. 1 is a sectional view of a cable according to the invention,comprising 12 cable modules of 8 fibers (the fibers are shown only inone of the modules),

[0037]FIG. 2 is a sectional view of a cable according to the invention(modules not shown), the external sheath of which comprises initiators,

[0038]FIG. 3 is a sectional view of a cable according to the inventioncomprising a double sheath (only one module shown),

[0039]FIG. 4 is a sectional view of the cable of FIG. 1 (only one moduleshown) after having opened an access window,

[0040]FIG. 5 is a sectional view of a cable according to the invention(modules not shown), the sheath of which has a lower thickness region,

[0041]FIG. 6 is a perspective view of a cable according to the inventionhaving two windows for access to the modules,

[0042]FIG. 7 is a perspective view of the cable of FIG. 6 after closingthe downstream window and putting in place a sleeve protecting thebranched module.

[0043]FIG. 1 shows a sectional view of a cable (1) according to theinvention. This cable comprises 12 modules (2) of 8 monomode ormultimode optical fibers (3), with a diameter of 250 μm, and coated witha thin film (4) which can be stripped easily. The dimension of themodules may range from 0.8 to 1.3 mm for 12 fibers. The modules (2) aresubstantially spread out in a sheet over the long axis xx′ of the innercavity (5) of a sheath (6), some modules overlapping. The sheath (6) isof general ovalized external shape. It comprises reinforcing members (7)of the nonmetallic glass-epoxy type located on each side of the shortaxis. The cavity (5) is of a general ovalized shape with a long axiscoincident with the long axis xx′ of the oval external shape. Themodules are placed so as to retain some clearance along the short axisyy′ of the cavity. The sheath comprises colored threads (8 a, 8 b) oneach side of the short axis.

[0044]FIG. 2 shows a section of a cable (1) according to the invention.In this embodiment, the sheath (6) forming an internal cavity (5)comprises reinforcing members (7), and fracture initiators (9 a, 9 b)colored at the base of the hollows (10 a, 10 b).

[0045]FIG. 3 is a sectional view of a cable (1) according to theinvention comprising an internal cavity (5) in which only one module (2)is shown, a sheath (6) made of PVC, and a protective jacket (11) of themedium- or high-density polyethylene type comprising reinforcing members(7) and colored threads (8 a, 8 b).

[0046]FIG. 4 is a sectional view of the cable (1) of FIG. 1 after awindow (12) for accessing the modules (2) has been opened in line withthe colored threads (8 a).

[0047]FIG. 5 shows a sectional view of a cable (1), the sheath (6) ofwhich has a region of lower thickness, between two guides (16). Thesheath forms an internal cavity (5) and comprises reinforcing members(7).

[0048]FIG. 6 shows a perspective view of a cable (1) according to theinvention, comprising a cavity containing modules (2) surrounded with asheath comprising reinforcing members (7) and colored threads (8 a, 8b), in which two access windows (12, 13) have been opened. A module (2)may be cut in the downstream window (13) before being extracted in theupstream window (12).

[0049]FIG. 7 is a perspective view of the cable of FIG. 5, in which thedownstream window (13) has been closed by adhesively bonding a lid (14),and the branched-off module (2) has been placed in a corrugated sheath(15).

[0050] It is therefore seen that the optical cable with continuousaccessibility, which is the subject of the invention, because of itspreferably oval general shape, because of its two favored axes one ofwhich is for easy curvature and accessibility to the modules spread outlengthwise in the long dimension of its central orifice and the other ofwhich is for a considerable clearance of the modules which alsofacilitates extraction and provides the cable with good thermal andmechanical qualities, offers considerable advantages which lead to veryeconomical cabling approaches of a centralized type either for the cableof buildings, or for small external networks or optical loops.

1. An optical cable with continuous accessibility (1), comprising aclosed protective sheath (6) surrounding a cavity (5) having in crosssection two substantially perpendicular axes intersecting at the centerof the cavity, and at least two optical fibers (3) arranged such thatthey occupy the greater part of the cavity along one axis but that theyallow a clearance along the other axis of the cavity.
 2. The cable asclaimed in claim 1, characterized in that the fibers are organized in atleast two optical modules (2) each one comprising at least one fibersurrounded by a thin film (4).
 3. The cable as claimed in claim 1 or 2,characterized in that the fibers or the modules are positionedlengthwise in the cable.
 4. The cable as claimed in any one of claims 1to 3, characterized in that one axis is longer than the other.
 5. Thecable as claimed in claim 4, characterized in that the fibers or modulesoccupy the greater part of the long axis.
 6. The cable as claimed in anyone of claims 1 to 5, characterized in that the cavity has a shape whichis circular or oval or ovalized, preferably ovalized.
 7. The cable asclaimed in any one of claims 1 to 6, characterized in that the sheathhas an external shape which is circular or oval or ovalized, preferablyovalized.
 8. The cable as claimed in claim 7, characterized in that thelong axis of the sheath coincides with the long axis of the cavity. 9.The cable as claimed in any one of claims 1 to 8, characterized in thatit comprises a jacket (11) around the sheath.
 10. The cable as claimedin any one of claims 1 to 9, characterized in that the sheath or thejacket has one or more colored threads (8 a, 8 b).
 11. The cable asclaimed in any one of claims 1 to 10, characterized in that the sheathor the jacket has one or more initiators (9 a, 9 b) which are possiblycolored (10 a, 10 b).
 12. The cable as claimed in any one of claims 1 to11, characterized in that the sheath or the jacket has a region of lowerthickness.